(a) Field of the Invention
The present invention relates to a focusing optical system for single-lens reflex cameras.
(b) Description of the Prior Art
There have conventionally been known some focusing optical systems which are described below:
One of such focusing optical systems comprises a semitransparent mirror 3 arranged on the optical axis and in the vicinity of a focusing glass 2 of a single-lens reflex camera as shown in FIG. 1, whereby a portion of the light emitted from an object to be photographed and having passed through a photographing lens 1 is split and an image of object formed on a detector element 5 is refocused with a auxiliary lens 4. This conventional method has a defect that the semitransparent mirror arranged in the vicinity of the focusing glass forms a dark portion at the center of the view-finder system, thereby hindering observation through the view-finder. There have also been known another focusing optical system for single-lens reflex cameras which comprises a semitransparent mirror 6a at the center of a quick return mirror 6 as shown in FIG. 2, and is so adapted as to deflect a portion of the light having transmitted through said semitransparent mirror downward or two right and left directions with another reflecting mirror 7 and further focus said portion of the light on a detector element 9 with a auxiliary lens 8. The focusing optical system mentioned as this example has defects that it requires numerous component parts which occupy large spaces for their arrangement and that arranging errors for individual parts are totalized, thereby degrading focusing accuracy.